Amplifier T cell activity can be transferred by spleen cells harvested 72 hrs after priming with Type III pneumococcal polysaccharide (SSS-III) and can be abolished by treating the transferred cells with monoclonal anti-Lyt 1, anti-Ia, or anti-Thy 1 antibodies in the presence of complement; thus, amplifier cells represent a distinct subpopulation of T cells. Amplifier T cells are radiation sensitive but insensitive to treatment with cyclophosphamide. When amplifier cells were transferred to athymic mice, the enhancement obtained was much greater than that produced in euthymic mice; this is due to the lack of suppressor T cell activity in euthymic recipients which enables amplifier T cell activity to be expressed more fully. Although the induction and activation of amplifier T cells, is antigen-specific, the product made by amplifier T cells may not be antigen-specific in its mode of action. Since amplifier T cells can be induced and activated by exposure to immune B cells, specificity is due in whole or in part to the ability of amplifier T cells - like suppressor T cells - to recognize the idiotypic determinants of B cell-associated antibody specific for SSS-III.